Linearly actuated valves used to control the flow of fluids are often actuated by solenoids. Solenoid valves pose several shortcomings. Typically, they are heavy, require high voltage and high current, and are expensive and difficult to manufacture, in part because tight component tolerances are required. Solenoid valves generate heat and create external magnetic fields which may affect other elements in a system or even prohibit the use of such valves in certain thermal or magnetic sensitive applications. Finally, precision control of the valve is difficult and the response time of the valve is slowed by the requirement of the relatively large mass of the plunger.
Several other U.S. patents namely, U.S. Pat. Nos. 4,577,607, 4,714,193, 4,790,343, 4,932,429, and 4,941,526, disclose valves which utilize shape memory alloy springs for actuation. The shape memory alloy members respond to ambient temperature, not electric current. In the valves disclosed in U.S. Pat. Nos. 4,577,607 and 4,714,193, a solenoid is used to override the valve position created by the shape memory alloy spring members. The valves in these patents have the common shortcoming of being unable to quickly and accurately control the extent to which the valve is open.
Applying an electric current to a shape memory alloy member to actuate a valve is disclosed in U.S. Pat. Nos. 4,736,587 and 4,973,024. The electromechanical valve disclosed in U.S. Pat. No. 4,736,587 uses two shape memory alloy springs controlled by an electric circuit. The springs can either be in a compressed or a relaxed state and are controlled by a circuit which provides current to either of the springs (but not both) to toggle an inlet port to either of two outlet ports. Therefore, continuous current is not necessarily applied to the shape memory alloy springs in this invention and the extent to which the valve is positioned between open and closed positions is not accurately controllable.
The manufacture of shape memory alloy springs is sometimes difficult as the element must be taught to be in such a coiled shape. Also, when subjected to an electric current, shape memory alloy springs may be susceptible to temperature gradients which may deform the coiled shape, and, therefore, portions of the shape memory alloy spring may not attain the proper temperature for phase transformation. Finally, coil-shaped memory alloys do not have as great a recovering force as linearly-shaped memory alloys, as the amount of deformation, and hence recovering force, of the spring is maximum only at the surface of the member and weakens toward the center.
The valve disclosed in U.S. Pat. No. 4,973,024 uses a shape memory alloy wire which is controlled by an electric circuit. Specifically, a contact member, such as the valve casing, is in contact with the metal plunger of the valve when the electric circuit is closed. The circuit is open when the valve plunger retracts from the contact member. During movement of the plunger, this metal-to-metal contact can result in sparking or arcing and, therefore, limited control of the valve. Control is also limited because continuous control, or current, is not applied to the shape member alloy after the plunger has moved enough to break the electrical circuit. U.S. Pat. Nos. 4,973,024 and 4,806,815 suggest the use of pulse width modulation for actuation to control the shape memory alloy members.